1. Field of the Invention
The present invention relates to a toner to be used in image forming equipment such as a copier, a laser printer and a facsimile, and to a non-contact developing method using the same. More particularly, it relates to a toner applied to a non-contact developing unit for visualizing an electrostatic latent image by flying the toner to an electrostatic latent image holder facing to the toner on a toner carrier with a gap by electrostatic force, and to a non-contact developing method using the same.
2. Description of Related Art
Hitherto, there has been known an electrostatic copier in which charged toner is carried on a toner carrier and the toner and an electrostatic latent image holder are disposed in non-contact from each other to develop an electrostatic latent image by electrostatic force acting between the toner and the electrostatic latent image holder (see Japanese Patent Publication No. 41(1966)-9475). The publication No. 41-9475 teaches that the non-contact developing method allows a copied image having no background fog to be obtained because the toner deposits only on the location which corresponds to an image portion of the electrostatic latent image.
However, when the non-contact developing method is compared with a contact developing method, the latter method can carry the toner to an electrostatic latent image portion mechanically, while the non-contact developing method is required to fly the toner by electrostatic force and is unable to assure sufficient development unless the electrical property of the toner and the developing conditions of the developing unit are fully optimized.
Accordingly, the above-mentioned publication No. 41(1966)-9475 teaches merely the basic idea of the non-contact developing method and discloses nothing about the property of the toner and the developing conditions, so that it is difficult to implement it.
As a case of color development in which a non-magnetic monocomponent toner is flied in a DC electric field, an article (1) entitled "One Drum Color Superimposing Process -DC Electric Field Flying-Development" has been published in the Journal of Society of Electro-photograph of Japan, Vol. 29, No. 1, 1990.
According to the article (1), the color development in which the non-magnetic monocomponent toner is flying-developed in the DC electric field has been put into practical use by reducing image-force, which is an adhesive force, acting on the toner laminated on a toner carrier to increase the property of the toner for flying from the toner carrier to an electrostatic latent image holder.
Further, in order to give a sufficient flying property, a non-magnetic monocomponent toner having a relatively large particle size of 12 .mu.m was used and a charge-to-mass ratio which is a quantity of charge per unit mass thereof was set at a low value of 1 to 5 .mu.C/g.
This is because a large toner charge-to-mass ratio was believed to increase image-force Fi and to decrease the flying property of the toner, thus considerably decreasing the developability, because the image-force Fi, which is an electrostatic adhesive force of the toner, increases in proportion to the square of the charge-to-mass ratio. Accordingly, it was necessary to increase the particle size of the toner because the small toner particle size would increase the specific area of the toner, thereby increasing the toner charge-to-mass ratio as well.
Further, because the non-contact development requires larger Coulomb force than the contact development, the flying property of the toner having a small particle size would be considerably decreased when it is applied to the non-contact development. Due to that, there has been a problem that the toner having a small particle size which should otherwise be very effective in improving an image quality cannot be used in the non-contact developing method. The toner in the non-contact developing method has been limited to those having a large particle size and having a small charge-to-mass ratio.
The non-magnetic monocomponent toner is used in the DC electric field flying-development because it allows toner images of a plurality of colors to be superimposed without color mixture and is suited for color development.
Further, a method for increasing the flying property of the toner by giving mechanical vibration other than the electrostatic force in a developing section has been proposed as a method for reducing adhesive force of toner on a toner carrier.
In a developing unit described in Japanese Patent Laid-open No. Hei. 5(1993)-232802, a method for increasing the flying property by providing a vibrating member in contact with a belt-like toner carrier to reduce the adhesive force of the toner on the toner carrier has been disclosed.
In a color image forming equipment described in Japanese Patent Laid-open No. Hei. 5(1993)-297711, a mechanical impact is applied to the developing unit when it begins to fly the toner so that the toner having a small particle size can easily fly.
Further, when the non-magnetic monocomponent toner is used, the toner cannot be fully conveyed unless the fluidity of the toner is good, because the toner cannot be conveyed by magnetic force.
Then, there has been known a method of adding another kind of particles to the toner for the purpose of improving the chargeability and fluidity of the non-magnetic monocomponent toner as disclosed in, for example, Japanese Examined Patent Publication No. Sho. 59(1984)-7098 entitled "Electrostatic Latent Image Developing Method" and No. Hei. 2(1990)-45191 entitled "Developing Method".
In the above-mentioned publication No. 59(1984)-7098, a monocomponent developer containing hydrophobic silica in toner is charged by triboelectric charging and is then supplied to a developing section. Thereby the fluidity of the toner is enhanced to prevent coagulation.
In the publication No. Hei. 2(1990)-45191, 1 to 50 parts by weight of granulating silica powder having 1 to 100 .mu.m of particle size is added into 100 parts by weight of insulating toner particle to improve a triboelectric charging performance of the toner.
Further, there has been known a method for carrying a toner having about 15 to 100 .mu.m of thickness and 0.1 to 0.6 g/cm.sup.3 of packing density on a toner carrier and flying-developing the toner through 100 to 500 .mu.m of development gap as disclosed in, for example, U.S. Pat. No. 4,666,814 and Japanese Patent Laid-open No. Sho. 60(1985)-87347. Still more, there has been known a method for carrying a toner having about 15 to 80 .mu.m of thickness, 0.1 to 0.6 g/cm.sup.3 of packing density and 3.times.10.sup.-10 .ltoreq..vertline.Q.vertline..ltoreq.10.sup.-7 of charge density Q(C/m.sup.2) on a toner carrier and flying-developing it through 100 to 500 .mu.m of development gap as disclosed in U.S. Pat. No. 4,666,815 and Japanese Patent Laid-open No. Sho. 60(1985)-87343 for example.
Further, there has been known a method for carrying a toner having about 30 .mu.m of thickness and 3 .mu.C/g of charge-to-mass ratio on a toner carrier and flying-developing it through 100 to 500 .mu.m of development gap as published in an article (2) entitled "Electrostatic Influence of the Toner Layer on the Photoconductor" in the Sixth International Congress on Advances in Non-Impact Printing Technologies, 1990, p. 34.
However, the flying-development using the toner having the large particle size and the low charge-to-mass ratio to improve the flying property thereof as described above has had a problem that it is apt to produce wrong sign toners (reverse polarity toners) and to cause background fog and a reduction of sharpness of edge, thus deteriorating the image quality.
This problem is outstanding especially when monocomponent toner is used. It is because the monocomponent toner is apt to produce a toner with the reverse polarity because it uses no carrier, whereas two-component toner is charged by friction between the carrier, having a charge polarity opposite to that of the toner, and the toner itself can be charged with a normal polarity. In particular, when monocomponent toner having a low charge-to-mass ratio is used the rate of the reverse polarity toner may reach to 30% in the toner to be developed.
Further, the method of developing the non-magnetic monocomponent toner in a DC electric field has had a problem that the toner layer is apt to be flown apart, as common to the non-magnetic toner. That is, while the non-magnetic toner is carried on the toner carrier mainly by image-force (electrostatic adhesive force) because it cannot be laminated and carried on the toner carrier by magnetic force like magnetic toner, the toner is apt to be flown apart because the toner having a small charge-to-mass ratio decreases the image-force, thus deteriorating the developability.
Although the method of developing the non-magnetic monocomponent toner in the DC electric field is suitable for color development, it has a number of disadvantages in terms of image quality as described above as compared to the conventional methods such as a two-component magnetic brush development. While a method of developing a black toner by the two-component magnetic brush development by using a toner having a small particle size and of developing only color toners by the non-contact developing method by using non-magnetic monocomponent toners having a relatively large particle size has been adopted sometimes as practical means for putting into use, it has had a problem that it complicates the equipment.
The toner having a large particle size has had a problem that a distance between a position of the center of gravity of the toner at the outermost surface of the toner carrier and an electrostatic latent image is separated, even though the development gap is constant, so that an electric field pattern of the latent image acting on the toner attenuates, thus decreasing a resolution of the image after the development.
Beside them, the non-contact development has had a problem of a phenomenon that a density at edge is emphasized depending on a development pattern due to the relation of the peripheral speed of the toner carrier with that of the electrostatic latent image holder.
Although the method disclosed in Japanese Patent Laid-open Publications No. Hei. 5(1993)-232802 and No. Hei. 5(1993)-297711 allow the toner having a small particle size to be used, they have problems such that the toner carrier is confined on a belt, separate means for applying mechanical vibration or impact is necessary and the equipment is complicated, thus increasing the cost.
Although the method disclosed in Japanese Examined Patent Publications No. Sho. 59(1984)-7098 and No. Hei. 2(1990)-45191 is effective in improving the chargeability and fluidity of the non-magnetic monocomponent toner by externally adding silica to the toner, it describes nothing about a correlation to the adhesive force acting on the toner, which is an important factor in the non-contact development.
Although the developing methods disclosed in U.S. Pat. No. 4,666,814 (Japanese Patent Laid-open No. Sho. 60(1985)-87347), U.S. Pat. No. 4,666,815 (Japanese Patent Laid-open No. Sho. 60(1985)-87343) and in the article (2) in the Sixth International Congress on Advances in Non-Impact Printing Technologies have a feature that the toner having less charge-to-mass ratio or charge density is carried on the toner carrier with a lower packing density and a thicker layer thickness, an experiment showed that the prior art developing method without considering the inter-particle force of the toner into account is not practical because its developability is remarkably inferior.
Actually, it has been found from the property of the toner, equations of the adhesive force and flying experiments that the adhesive force which acts on the toner laminated and carried on the toner carrier includes an adhesive force called the inter-particle force Fv other than the electrostatic adhesive force called the image-force Fi and that it is important to suppress the inter-particle force Fv, other than the image-force Fi, which act on the toner in non-contact developing. It has been also found from the experiment that the flying-development can be implemented fully with toner having a large charge-to-mass ratio regardless of the particle size thereof by reducing the inter-particle force Fv other than the electrostatic force.
Therefore, the toner having a small particle size which is very effective in improving the image quality may be adopted in the non-contact developing method by suppressing the inter-particle force and defining the size thereof, and thus the above-mentioned problems of the prior art can be solved.